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Concentrator Photovoltaics: The Next Step Towards Better Solar Power

Today’s concentrator photovoltaic (CPV) technologies have shown promising potential for more efficient solar power. The latest systems are said to be capable of handling the power of a hundred suns. Yet prototypes have failed to compete with cheaper flat panel solar systems that dominate the market. The U.S. Department of Energy’s Advanced Research Projects Agency (ARPA-E) is determined to push CPV to the next level. On 24 August, at the Clean Energy Summit, U.S. President Barack Obama and Energy Secretary Ernest Moniz announced a program called MOSAIC that will invest $24 million into CPV solar technology development.

Why can’t today’s CPV systems compete? The concentrators can only convert direct sunlight into energy, missing out on the large fraction of sunlight diffracted by clouds and the atmosphere. Manufacturing costs of concentrator apparatuses have also prevented CPV from reaching mass production.

That’s where the MOSAIC initiative comes in. The 11 new CPV programs under MOSAIC’s umbrella are investigating an array of system designs to address cost-efficiency and performance challenges. The list of projects include economical micro-PV cell construction, waveguiding solar concentrators, and single-junction cells that will maximize concentration under indirect and diffuse sunlight.

“ARPA-E is supporting new technology that can help the industry progress even more, but even where it is today is quite exciting,” says Sarah Kurtz, a research fellow working on CPV technology (separately from the MOSAIC effort) at the U.S. National Renewable Energy Laboratory (NREL) in Colorado.

For a large-scale commercial flat plate solar panel system, efficiency is approximately 16 to 20 percent, while a typical CPV system is 25 to 30 percent. In engineering labs, efficiency test results show that the gap between CPV technology and flat planel photovoltaics is even greater. Research groups have created CPV cells that convert more than 40 percent of the light that strikes them to electric current—the highest marks received in testing environments. Three of these groups’ systems have even passed the 46 percent mark.

“This means that these results are very repeatable,” says Keith Emery, a principal scientist who measures solar cell efficiency at the National Center for Photovoltaics at NREL. “I wouldn’t be surprised that by the next two or three years, an individual research group will reach 50 percent efficiency. Fifty percent is a realistic goal that people have on the drawing board.”

CPV maximizes efficiency by using multiple optical elements such as mirrors and lenses to reflect light into a super concentrated beam that is aimed at a solar cell. Machinery adjusts the panels throughout the day so that the cells are exposed to a maximum amount of direct sunlight. The technique is similar to the way you would move a magnifying glass while burning your name into a piece of wood, explains Kurtz.

The optical elements make it possible to use smaller, higher performance solar cells. The miniaturized cells make it easier to modulate their movement to prevent overheating and degradation. Some labs have even constructed cells as small as 1 square millimeter; they can generate more power in less space than flat panels can.

The next push is to make CPV materials that are even greener and cheaper than flat plate photovoltaics. Engineers are testing mirrors constructed from recyclable plastic, with aluminum-based reflective coatings, says Kurtz.

As the energy industry slowly transitions old fossil fuel plants into photovoltaic and other renewable power plants, solar energy can become a larger part of the electric grid.

“There is a rumor that solar is the technology of the future,” Kurtz says. “It has grown a lot, but it is still a minuscule part of the electricity we use.” While photovoltaics provide only about 1 percent of the electricity generated in the U.S., the rate of solar installation is increasing every year. “If we maintain industry growth rates long enough, solar could be at something like 25 percent of the world’s electricity production,” says Kurtz.

Why You Probably Don't Care About a Fuel Cell iPhone That Can Run for a Week

A smartphone powered by a fuel cell that can run for an entire week without recharging sounds absolutely amazing. The Telegraph is reporting that a British fuel cell company called Intelligent Energy has managed to stuff a fuel cell inside of an iPhone 6, allowing the phone to run for an entire week on a single charge.

Sort of.

As with anything that sounds absolutely amazing, it's not that simple, and the truth is likely not something that's worth getting excited about at all.

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Ultrathin Solar Cells for Lightweight and Flexible Applications

Photovoltaic cells are finding a host of new applications, even powering airplanes. An example is the Solar Impulse 2 plane, which is blanketed by photocells that can keep it airborne indefinitely.  Although their conversion efficiency, at 22 percent, is comparatively good, covering 200 square meters with the 130-micrometer-thick cells adds significant weight to the plane.

A team of scientists from the Johannes Kepler University Linz in Austria reported in the 25 August edition of Nature Materials Online that they have created prototype solar cells that are a mere 3 micrometers thick. 

“We are interested in developing a solar cell technology that is the most lightweight and has the highest  power per weight that is possible in a research lab,” says physicist Martin Kaltenbrunner, the spokesperson for  the team.  Although the ultrathin cells’ conversion efficiency is 12 percent, they weigh about 100 times less than the lightest solar cells currently available. The researchers reached an interesting milestone: A square meter of solar cell weighing 5.2 grams produced 120 watts. “It is an absolute record in power per weight,” says Kaltenbrunner.

For their light harvesting material, the Austrian researchers used organolead halide perovskite, a hybrid organic material that is now viewed as a promising alternative to silicon. Says Kaltenbrunner:

The neatest thing about the perovskites is that they are direct band gap materials and can be made very thin, but can still absorb a lot of light. So a couple of hundred nanometers are actually enough to collect all the photons in the absorption range that it has and convert them to electron-hole pairs.

For the creation of these thin perovskite layers, the scientists used solution processing to deposit the thin layer on the transparent electrode. “We used that technique but we had come up with a new method to get a nice pinhole film on a very rough substrate,” says Kaltenbrunner. “Our foils are very thin but also very rough,  and it is challenging to process nanometer thick layers on them without having too many defects.” The other, non-transparent electrode, which could be either gold, aluminum  or chrome, also serves to increase the efficiency of the cell by reflecting back the photons that still passed through. The total thickness of the device, including the two 100-nanometer-thick electrodes, the substrate, and protective layers is 3 micrometers.

The researchers’ processing tools allowed the creation of films only as large as about 10 centimeters, but in the future, roll to roll processing should be possible, says Kaltenbrunner.

At this stage of the research, the device is vulnerable to oxygen and water, but functions at full capacity with artificial sunlight for several days. The team says it made demonstrators that operate in air, and that a half year later, the gadgets are still functional. “We had this little zeppelin and a few days ago we tried it, and [the solar panel] still propels it."

Because these layers are so thin, the film is very flexible and elastic, which results in other interesting advantages besides low weight. By creating the foil on a stretched substrate and then releasing the tension, the deposited foil wrinkles up. “You get this microtexture of tiny valleys that collect more light and, per unit area, we get a higher efficiency,” says Kaltenbrunner.  The flexibility provides yet another advantage: “You fabricate on a large scale in a flat geometry a very thin electronically functional foil, and then you laminate it to whatever object you want,” Kaltenbrunner points out.  “The car industry is very interested in this.”

Photovoltaics that are almost weightless may end up in everything that flies, such as weather balloons, small unmanned planes, and even drones, says Kaltenbrunner.

What about applications in space? It is still too early to answer this question because it is not known whether the film could withstand the intense particle radiation. “It would be really exciting to have partners on board that can test this, like ESA or NASA,” says  Kaltenbrunner. For example, one could have solar sails that might also be solar cells. “Interestingly, this was the driving force to thin down solar cells and make them lightweight.” Kaltenbrunner explains. “This started in the 1980s; there were papers on silicon solar cells on plastic substrates just for this purpose.” 

Supercritical Carbon Dioxide Can Make Electric Turbines Greener

The U.S. Department of Energy (DOE) is on the hunt for technologies that can support a smarter electric grid. It is currently devoting millions of dollars, via its SunShot Initiative, to create more efficient photovoltaic systems. But in addition to the solar power that is SunShot's focus, the DOE it is looking to improve conventional electric power generation. Sandia National Laboratories in Albuquerque, New Mexico, along with its new partners, has received $8 million to make advances in supercritical carbon dioxide gas turbines.

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Synchronization Controls Could Help Smooth Microgrids

Circadian rhythms and microgrids might not seem to have much in common, but in the world of theoretical mathematics, they do.

New research published Friday in Science Advances suggests that the same mathematical models that help scientists better understand and explain biological phenomena could also be applied to making small, islanded power grids run more efficiently.

Microgrids come in various flavors, but usually they are localized grids that can disconnect from the larger power grid and operate independently. More recently, microgrids have combined clean energy generation and more traditional generation (such as diesel or natural gas turbines) to deliver both heat and electric power. They manage current with energy storage and controls.

Because the grids are small, they’re prone to more severe fluctuations in voltage and frequency than are larger grids, which can more easily smooth fluctuations across their wider systems.

This is where a well known mathematical model for synchronization, the Kuramoto model, can help, says Per Sebastian Skardal, assistant professor of mathematics at Trinity College and lead author of the paper.

The Kuramoto model is a phase oscillator model that defines each oscillator as just a phase angle. The behavior of each phase depends on its interaction with the other phases, explains Skardal.

The model has helped to explain the synchronization of various processes, including the rhythmic flashing of fireflies and neurons firing in the brain. “With the power grid, on the other hand, we are going one step further and using what we know about networks and synchronization to make deliberate choices to improve the functionality of a given system,” says Skardal.

Skardal and his collaborators found that in islanded microgrids that are disconnected from the large power grid, there are essentially a few problematic oscillators. They tend to prefer a frequency that is either much higher or lower than the other oscillators in the network, or they’re loosely coupled to the network. These problematic oscillators could be a set of solar panels that have widely variable output, or a large power draw that turns on and off suddenly.

Skardal says the model should apply regardless of the microgrid configuration. Although the work is firmly theoretical at this point, Skardal and his collaborator Alex Arenas, professor of physics at Rovira i Virgili University in Spain, are interested in applied mathematics, which would take further studies and engineering work.

The model would ultimately inform control systems for microgrids, giving the ability to identify the problem oscillators and adjust them in real time. Preventing or minimizing the grid fluctuations within a microgrid could potentially reduce costs because fewer inverters would be needed or a more simplified and standardized control scheme could be implemented.

Whether this research would apply to a microgrid when it is connected to the main power grid, or help to limit fluctuations in larger power grids whose stability is affected by the variability of inputs from renewable energy sources, is yet to be seen.

Skardal says he believes both may be future applications of the current research, but it is too early to say for sure. In the case of a microgrid that is not islanded, “I believe that the intuition behind the idea will hold,” says Skardal. But the Kuramoto network model would have to be adjusted to take some external forcing, which would be the influence of the larger grid, into account. 

For large power grids with high levels of renewable energy generation, the complexity of the system would make the model more complicated, but the same math could potentially apply.

But power grids aren’t the only things that could benefit from this line of research, the authors argue. “We hypothesize that our findings here may potentially shed some light on the control of synchronization in other contexts,” they conclude in the paper, “such as cardiac physiology and neuroscience.”

New Mapping Tools Show Just How Bad China's Air Pollution Really Is

By crunching data from satellites and ground monitoring stations, environmental scientists are creating maps and forecasts that reveal the scope of China’s air pollution problem in unprecedented detail. The big question is: Will all this data have any impact on environmental policy?  

The maps show that China’s air pollution is beyond bad, it’s catastrophic. In Beijing, residents are responding to the ongoing “airpocalypse” by wearing heavy-duty respirator masks as they go about their daily business, or, increasingly, by never going outside. Fancy schools now feature domes that enclose their playgrounds and sports fields, and residential towers connect directly to underground malls and subway stations. 

At the time of this writing, Beijing’s air quality index is 159, which is actually a pretty good day for the megacity. True, the widely used EPA rating system calls anything above 150 “unhealthy,” likely to aggravate heart and lung conditions and to cause respiratory problems among the general population. But there have been days when Beijing’s air quality is literally off the charts, exceeding the “hazardous” rating that tops out at 500. 

A number of existing websites and apps provide real-time air quality info for worried city residents wondering if they dare cycle to work or open their windows. But precise data for air quality across the country has been hard to come by, and both forecasts and historical data have been lacking. A startup and a nonprofit, both hailing from the San Francisco area, are now addressing those gaps.

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Google Introduces Project Sunroof

Does it make sense to install solar panels on your roof? You probably have no idea. But as of today, Google knows. The colorful and recently alphabetized search monstrosity has launched a new tool called Project Sunroof. It will use data you may not have realized that Google even had to tell you how much money you can save by turning your roof into a photon harvester.

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Hourly Model of Air Pollution Can Reduce Health Costs

Fossil-fuel-burning power plants can reduce the impact their pollution has on air quality and human health by controlling how active they are during certain hours, scientists at Georgia Institute of Technology say. These findings could help limit the drawbacks of generating electricity from fossil fuels without additional investment, they add.

Burning coal and other fossil fuels releases pollutants such as sulfur dioxide and nitrogen oxides. Prior research suggested that air pollution causes 200,000 early deaths each year in the United States alone.

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MIT Has Plans for a Real ARC Fusion Reactor

The Marvel movie version of Tony Stark graduated from MIT in the early 1990s. He built an ARC reactor at Stark Industries later on, but apparently, some of the initial research he did as an undergrad stuck around in some notebooks somewhere on a dusty shelf at MIT. It took them only a few decades, but a team of MIT researchers has been able to develop tentative plans for a fully armed and operational ARC fusion reactor of their own.

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Supercomputer Network Simulates Material That Might Not Melt in a Sunspot

Computer simulations can save time and money when investigating new technical designs but also when looking for new materials. Some serious supercomputing helped two scientists at Brown University in Providence, RI, to—virtually—break a melting-point record.

The current record is held by a mixture of hafnium, tantalum, and carbon (Ta4HfC5), which melts at 4,215 K (3942 oC).  The Brown scientists predicted that a material made up from a mixture of hafnium, nitrogen, and carbon, could have an even higher melting point, 4,400 K (4127 oC), which is about two thirds the temperature at the sun's surface. (Sunspots range from 3000 – 4500 K, so such a material would probably stay solid in one.) At that temperature, the theoretical material would emit light with an intensity about one third of the sun’s surface. The researchers, Axel van de Walle and Qi-Jun Hong, published the results of the computer simulations of this compound in the journal Physical Review B on in June.

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